Use same MANAGER_TOPIC variable

In ironic/conductor, manager.py and rpcapi.py both specify the variable
MANAGER_TOPIC = 'ironic.conductor_manager'.

They have to have the same value because this is the rabbit topic that
they communicate over. It would be less error-prone (and more obvious
that they should be the same), if it was only specified in one place.

Since cmd/conductor.py uses MANAGER_TOPIC from manager.py, rpcapi.py
was modified to do the same.

Change-Id: I5875ffb7548c1b5079ac4221ce1000ae620509d8
Closes-Bug: #1268810
Merge "Use same MANAGER_TOPIC variable"